Since the advent of telephony, there has been the need for switching and transmitting information from one telephone subscriber to another where each subscriber's information is carried over a subscriber channel. Such switching was originally accomplished through manual cable connections wherein an incoming channel from a first telephone subscriber would be manually switched at a central location (central office) by connecting that subscriber channel to a desired outgoing channel corresponding to a desired second subscriber. Manual interconnections were later automated through use of a matrix of mechanical switches commonly referred to as space switching.
Initially, transmission between switching points was accomplished over transmission lines wherein each set of lines served one subscriber at a given instant of time. With the discovery of frequency division multiplexing (FDM) the transmission of multiple subscriber's signals over a single telephone line pair was accomplished between switching points. With frequency division multiplexing, each subscriber's signal is presented on the line in a different frequency bandwidth which therefore provides for simultaneous transmission of multiple subscriber signals on a transmission line pair simultaneously.
Mechanical switches presented problems due to their relatively large size and the maintenance associated with mechanical devices of that nature. Similarly with frequency division multiplexing, signals had to be modulated in order to present them on different frequency bandwidths prior to leaving a switch point and then had to be demodulated for switching upon arrival at the next switch point prior. Such modulation - demodulation procedures created there own electronic problems such as signal distortion.
Subsequent to the introduction of frequency division multiplexing, pulse code modulation (PCM) was developed for use in telephony. In pulse code modulation, the magnitude of the signal representative of the telephonic information is sampled and each sample is approximated to a nearest reference level. Such a procedure is generally referred to as quantizing. Following this operation, a code is transmitted in digital form to a distant location wherein the code is representative of the telephonic signal's magnitude at a given instant of time. The use of pulse code modulation effectively merged the functions of both switching and transmission. Signals were encoded at the source, passed through one or more switch points and then decoded at their destination. The circuitry associated with PCM was less complicated
that associated with FDM and furthermore, PCM allowed the replacement of mechanical switches with digital electronic switches. The use of PCM coding and digital switches gave rise to space and time switching. More particularly, space switching was accomplished through an electronic matrix of switches while time switching took advantage of the PCM inherent time division multiplexing by using an electronic time slot interchanger.
Current time slot interchangers are generally used in digital central office switches and digital crossconnects. The function of the time slot interchangers (TSI) allows crossconnection of subscribers while each subscriber's signal is maintained in its digital PCM form. More particularly, switching from one subscriber to another is accomplished by storing an incoming PCM signal from one subscriber's channel and having that signal subsequently sampled by a second subscriber's channel. Such switching from one subscriber to another is therefore effectively done through time switching and necessarily requires the use of memory for the storage of the PCM data of the first subscriber for reading by the intended second subscriber.
In the past, TSI's have been limited primarily due to limitation of memory technology. These limitations have generally been with regard to the speed of operation (that is the length of time necessary to write data into memory and to read data from memory), power requirements for the memory, physical size of the integrated circuit technology, and the number of external electrical interconnections necessary to the memory device(s).
Due to these limitations, large TSI functions were typically accomplished through use of a plurality of printed circuit board assemblies, each operating as a serial stage. Such multiple state configurations create complex switch control algorithms for utilization of all possible connection paths so as to minimize the possibility of call blocking; that is, failure to interconnect one subscriber to a desired second subscriber. The complexity of such switch algorithms have led to problems caused by limitations of th computer processors used therewith.
In some prior art implementations of TSI's, multiple board assembly stages could not be used for reasons of cost, physical size, delay requirements, interconnect sizing problems, or power constraints, all of which force features of such TSI's to be sacrificed. In some configurations, call blocking was allowed on a finite statistical basis which consequently limited the ability to interconnect any channel to any other channel under all circumstances. Furthermore, functions such as broadcast interconnects used for sharing common equipment such as tone, alarm code, or pseudorandom code generators and the like, have in some circumstances been sacrificed in prior art TSI configurations. In addition, maintenance functions such as loopback, bridging and terminating have been sacrificed. Furthermore, due to the unpredictability of the number of serial stages necessar to accomplish a given switch function, equal propagation delay switching of groups of channels is not achievable with these prior art TSI's.
The present invention is a time slot interchanger which can implement these functions without the use of multiple serial stages. The TSI according to the present invention performs such functions through use of a parallel matrix-type architecture, using high speed memory and large scale integration technology. Through use of a modular matrix architecture, full functionality, including loopback, broadcast functions and the like, is achievable for all individual channels. The parallel TSI matrix further provides for modular reduction in order to customize the architecture for particular functional needs. Its implementation with high speed memory and large scale integration technology reduces the size, cost and power requirements by approximately an order of magnitude in comparison to prior art TSI designs.
The simple matrix architecture of this parallel TSI simplifies the processor switch control algorithms since complex connect maps are no longer required. Furthermore, the connect memory associated with the switch blocks forming the matrix architecture of the present invention substantially simplifies the processor algorithms necessary to update the connect memory and consequently, the processor through its reduced overhead is able to perform other tasks or allowed to control more subscriber interconnections per unit of time.